Out of the snake pit

Airports - Giant 'supertrusses' feature on the new Bangkok airport. Report and photographs by Adrian Greeman

It began nearly 20 years ago with cobras in the spoilheaps and dragon lizards warming themselves on the tarmac of the site road; moved on to an imported Dutch world of watery polders and canals, and finally to a 'Dante's inferno' of heavy welding and steelfixing.

Sparks and smoke are now belching from a row of fiery furnaces for runway asphalt.

But amid tropical heat and monsoon rains, Thailand's new airport is finally taking shape just south of Bangkok. It might even get somewhere near a much heralded September opening this year, at least with a token plane-landing and formal ceremony, although there will still be a few months of work to achieve final international approvals.

The project very nearly did not happen at all. Despite the Thai government acquiring the land with great foresight 30 years ago - no small achievement in a country without compulsory purchase powers - and then planning, architecture and structural design work in the early 90s, everything hit the buffers in 1997, as the Asian currencies collapsed. Suddenly nothing moved on Bangkok construction sites. Although ground clearance and surcharging continued for a while, eventually the airport work stopped too.

Gradually, things have picked up, not least the airport, given a massive Baht73bn (US$1.86bn) injection by the Japan Bank for International Co-operation, some 60% of a total $3.18bn total project cost. Another $763M of private investment is being made.

Work restarted in 2002, with a Japanese dominated construction project direction team led by Pacific Consultants International, instead of the original group of Louis Berger and NACO.

But the original US architectural design by Murphy/ Jahn and TAMS with local firm ACT consultants is still being used. And most of the design and supervision teams who worked on the terminal and pavements are still in place.

Chief among them the TCS consortium led by Germany's Dorsch Consult with Scott Wilson, JAL and 10 Thai consultants.

The site is dominated by the huge terminal building and its tubular concourse extensions, stretching outwards in an H-shaped plan across the airfield.

'Extraordinary and exciting' structural work has been needed, says Scott Wilson's Richard Bartlett, who advised on and supervised the steel for the terminal roof and concourses.

Eight huge supertrusses form the flat 85,500m 2 'plain' of the roof, the main ones weighing around 1500t with two slightly smaller 1200t end units which carry less load. 'The steel quantities for the job are incredible, ' says Bartlett.

'There was 40,000t used just for the temporary works, plus more for the whole structure.' These 202m long trusses have a triangular structure with two top elements, themselves triangular and a lower chord. This reverses at the ends where the trusses cantilever beyond their sturdy support pylons and the compression and tension forces switch around. The central clear span is 126m.

Bartlett believes the structural effect is also fairly satisfying architecturally, with a faintly Thai feel to it. The twisted triangle shape is reminiscent of the temple dragons and demons.

The big pylons, 40m tall, comprise four square steel legs tapering into a 30m high waisted point where a complicated transition structure sits, before four shorter, outwardly tapering legs bring it to the full height. Steel fabricator and erector Yongnam Engineering from Singapore lifted the legs in.

'This and later work used some very large cranes, ' says Bartlett. 'And because of the soft marine clay ground in Bangkok we had to put in piling to support their operations.' The 20m deep soft clay meant piling was also necessary for most of the temporary steel frames.

Legs were welded insitu - 'I don't think there is any bolting work on the structure, ' says Bartlett - and there was plenty more welding at ground level for various components, not least joining the 12 modules delivered for the supertrusses.

'Welding 100mm thick plate was heavy work, ' says Bartlett.

To make sure welds were good the steel had to be pre-heated to 100¦C, which meant an hour of torching beforehand; insulating blankets were used to slow the cooling and prevent heat stress.

Each supertruss was strand jacked into position. Specialist VSL used the completed pylon and a separate temporary tower to support the jacks, which lifted a support beam under the truss.

Once at the top this acted as a rail to slide the truss onto the pylon, and then settle it into place using a variety of other jacks.

Parallel secondary trusses sit between each of the supertrusses carrying skylight glazing. These were lifted as modules onto an end platform where they were fully welded and fitted with purlins and then jacked along towards the centre one by one.

Main contractor, the ITO JV of Italian-Thai Development, with Takenaka and Obayashi from Japan, is now working on the entirely separate, concrete framed, four floor building underneath, which has no structural contact with the roof.

Unusual features include an embedded under-floor water cooling system.

Meanwhile, steel for the 25m high tubes of the concourses is just finishing. This is smaller but more complex, with a series of five-pin arch-truss 'hoops', 50m wide at the maximum over a 40m wide concrete base. Where two tubes intersect there will be a more complex area which will be used for airside retail and restaurant space.

With the terminal steel complete Bartlett is supervising work on one of the main hangars and several support buildings. Scott Wilson also designed the frame for Thai Airways' hanger (right), which is a vast steel box with a 270m long entrance capable of taking three of the new Airbus A380 'Superjumbos' side by side. The building width is 90m.

'The space inside needed to be as flexible as possible to park many smaller planes too so only two columns are allowed, ' says Bartlett. A 'cantilevering space frame' was the solution, with two main spines over the columns and the long door beam supported at the end. The back of the building is tied down with tension piles and concrete counterweighting.

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